Providing insecticide-treated underwear to people in homeless shelters was effective in eliminating body lice infestations, but the effect did not last and resistance to insecticide resistance increased, according to the results of a clinical trial by Samir Benkouiten, MPH, of Aix Marseille Université, France, and colleagues.

Body lice are contagious and can be spread through body contact, shared clothing, shared bedding and overcrowded conditions. Researchers sought to determine whether long-lasting, insecticide-treated underwear would protect against the proliferation of body lice in the homeless, according to the study background.

The study randomized 73 homeless people to underwear treated with the insecticide permethrin (n=40) and placebo (n=33). Follow-up visits were scheduled on days 14 and 45.

More homeless people with the insecticide-treated underwear were free of body lice on day 14 (11 of 40) compared with the placebo group (3 of 33). But that difference was not sustained on day 45 and was accompanied by increasing resistance in body lice collected from the homeless.

In conclusion, this trial clearly demonstrates that the use of permethrin-impregnated underwear had the consequence of increasing the percentage of permethrin-resistant body lice in sheltered homeless persons. These findings lead us to recommend avoiding the use of permethrin to treat body lice infestations, although implementing new strategies is crucial, the authors conclude.